Since a gallon of water weighs 8.354 pounds, and you use 11.81 gallons, it is very easy to find out how many he used. All you have to do is multiply 11.81 by 8.354 because you have 11.81 gallons and a gallon weighs 8.354 pounds. So when multiplied you get 98.66074 pounds. So Simon used 98.66074 pounds of water.